A refraction measuring instrument which is widely used at present normally includes a chin rest on which the chin of a subject is placed and a head rest to which the forehead of the subject abuts (see, for example, JP 11-299733 A) in order to prevent a relative position difference between the subject and the refraction measuring instrument. In such a refraction measuring instrument, the measurement is performed on the subject in an unnatural posture in which the subject slouches and the chin and the forehead are in contact with the chin rest and the head rest, respectively. Therefore, physical and mental burdens are placed on the subject. In addition, it is difficult to accurately measure the refraction of an eye to be examined in actual life.
In recent years, the development in the fields of image display devices, such as the development of three-dimensional displays is remarkable. A novel image display device achieves various effects by taking advantages of the structure of the human eye and the action thereof. In the example of the three-dimensional display, a stereoscopic image is to be displayed on a screen based on a parallax between the right eye and the left eye.
According to such a visual effect, sort of impractical operation may be forced to the eyes, so that the burden on the eyes becomes very large. Therefore, in order to display an eye-friendly image in view of human engineering, further research and development are proceeding at the present time. Thus, an instrument for measuring an accommodation state of the eyes viewing an image is required.
The refraction measuring instrument requires various conditions. Firstly, it is necessary to measure the refraction of each of the eyes to be examined while the eyes visually recognize the image displayed on the image display device. Secondly, the image display device is used in actual life, so it is necessary to perform the measurement in the natural posture based on the actual life. In the conventional refraction measuring instrument as disclosed in JP 11-299733 A, it may be difficult to satisfy this condition as described above. Thirdly, a person viewing the image does not necessarily stay in a predetermined position, so it is preferable to employ a structure capable of performing the measurement even when the person is moving from one place to another. However, a refraction measuring instrument having such a function has not been disclosed up to now. Fourthly, the image displayed on the image display device is a moving picture in many cases, so it is suitable to be able to continuously measure the eyes viewing the moving picture. An example of a refraction measuring instrument capable of performing real time measurement is disclosed in JP 2000-262475 A. In the refraction measuring instrument described in JP 2000-262475 A, it is difficult to satisfy the third condition in view of size thereof.
As a result, according to the refraction measuring instrument which is currently used, it is concluded that it is very difficult to achieve a purpose for making the assessment of the image display device.
Therefore, the present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide a refraction measuring instrument capable of measuring the refraction of an eye to be examined while a subject is viewing an object outside the instrument, such as the surrounding ambience of the subject or an image.
Another object of the present invention is to provide a refraction measuring instrument capable of measuring the refraction of the eye to be examined in a more-natural posture.
Another object of the present invention is to provide a refraction measuring instrument capable of measuring the refraction even when the subject is moving.
In addition to the two objects, another object of the present invention is to provide a refraction measuring instrument capable of measuring the refraction in real time.
It is to be noted that the refraction measuring instrument according to the present invention can be suitably used for the assessment of an image display device such as a three-dimensional display in addition to normal measurement for prescribing spectacles or contact lenses.